Bearing after the heat treatment quality problems often

A soft spot

Because insufficient heating and cooling is bad, improper operation and other causes hardening of the bearing surface of the part of the phenomenon known locally insufficient hardness quenching soft spots. It is the same as the surface decarburization can cause surface wear resistance and fatigue strength in serious decline.

Roller bearing parts in the quenching process due to the formation of stress cracks called quenching cracks. Cause such cracks reasons: because the quenching temperature is too high or too fast cooling, thermal stress and metal mass volume change when the stress fracture resistance than steel; work surface defects original (such as surface micro-cracks or draw marks) or steel internal defects (such as slag, severe non-metallic inclusions, white point, residual shrinkage, etc.) at a stress concentration quenching; severe surface decarburization and carbide segregation; parts after quenching and tempering insufficient or timely tempering; previous process caused by too much stress cold punching, forging folding, turning deep marks, oil, sharp edges, etc. ditch. Quenching cracks deep and slender, straight fracture, fracture surface without oxidation colors. The organization is characterized by quenching cracks no cracks on both sides of decarburization phenomenon, the obvious difference with forging cracks and material cracks. In short, resulting in quenching cracks may be due to one or more of these factors, the existence of internal stress cracks forming the main hardening. It is often in the bearing rings on the straight longitudinal cracks or ring cracking; ball bearing on the shape of the S-shaped, T-shaped or ring.

Second, due to heat

Three, overheating

Fourth, quenching cracks

Bearing parts in the heat treatment process, if it is heated in an oxidizing medium, the surface oxidation occurs to parts of the surface carbon content reduced, resulting in surface decarburization. Determination of surface decarburization depth available in metallographic examination and microhardness metallographic method. Surface decarburization depth over the last machining allowance will make parts scrapped. In the surface layer microhardness distribution curve measurement prevail, do arbitration criterion.

Five, surface decarburization

Six, heat deformation

Quenching temperature is low or poor cooling in the microstructure produced more than the standard prescribed troostite organization called due to heat tissue, which decrease in hardness, wear resistance drastically reduced, affecting bearing life.
Bearing parts during heat treatment, the presence of thermal stress and stress, this stress can be superimposed on each other, or partially offset, is complex, because it along with the heating temperature, heating rate, cooling, cooling rate, part shape and the size of the change, so the heat distortion is inevitable. Understand and grasp its variation can make the bearing part of the deformation (such as elliptical rings, rose and large size) placed controllable range, is conducive to the production of conduct. Of course, during the heat treatment the mechanical collision of parts also cause deformation, but this deformation can be used to reduce and improve operations avoided.
Port from the bearing part can be observed on the rough microstructure after hardening overheating. But to determine the exact extent of overheating must observe the microstructure. If GCr15 steel hardened tissues appear coarse acicular martensite, was quenched overheating tissue. Even cause serious overheating quenching cracks. As quenching overheated steel coarse crystals, may cause parts of the decrease in toughness, impact resistance decreased, the bearing life is reduced. Overheating tissues residual austenite increases, dimension stability decreased. Formation may be due to the quenching temperature is too high or too long causing heat insulation comprehensive overheating; also may be due to the original organization carbon ribbon severe in the two areas with the formation of carbon between the local martensite needles thick, caused by local overheating.

      Several bearing the above heat treatment process recurrent problem, which directly affect the quality of the bearing, so that the production process of the bearing, should try to avoid such a situation.